1. Field of the Invention
The invention relates to a motor control system and, more particularly, to a motor control system that executes drive control of an alternating-current motor by applying alternating-current voltage converted by an inverter from direct-current voltage stepped up by a converter.
2. Description of Related Art
In a related art, there is known an electric vehicle that includes an electric motor as a driving power source. The electric motor is driven by electric power from a battery to output power. A three-phase synchronous alternating-current motor may be used as the electric motor. The three-phase synchronous alternating-current motor is driven by the application of three-phase alternating-current voltage converted by an inverter from direct-current voltage supplied from a power supply.
In addition, in some electric vehicles as described above, the direct-current voltage supplied from the battery is not directly supplied to the inverter but is stepped up by a buck-boost converter according to a predetermined command value and then input to the inverter. it is advantageous to step up a system voltage with the use of the buck-boost converter to increase the system voltage in this way because it is possible to drive the alternating-current motor at higher torque and higher rotation speed.
Sinusoidal pulse width modulation (PWM) control, overmodulation control and rectangular wave control are known as a control method for the three-phase alternating-current motor. These control methods are selectively switched and used on the basis of a driving condition of a vehicle, a modulation factor (described later), and the like, widely.
For example, Japanese Patent Application Publication No. 2006-311768 (JP 2006-311768 A) describes a motor drive system controller that, in a predetermined control mode in which a modulation factor is not fixed, sets a target modulation factor such that a loss in the system is reduced as a whole and that variably controls a system voltage that is a voltage stepped up by a converter so that the modulation factor becomes the target modulation factor.
In addition, Japanese Patent Application Publication No. 2010-172156 (JP 2010-172156 A) describes that, in a motor control system for a vehicle, when a voltage phase reaches a threshold while rectangular wave control is being executed with the use of a battery voltage, the step-up operation of a converter is started.
As in the case of the motor control system described in JP 2006-311768 A, in a motor control system that includes a converter, an inverter and an alternating-current motor, it is advantageous to decrease a voltage stepped up by the converter to operate the alternating-current motor in rectangular wave control, that is, so-called single-pulse control, in order to reduce a switching loss in the converter and the inverter. However, because rectangular wave control is voltage phase control under field-weakening control, a motor loss increases as a field-weakening current increases. On the other hand, when a voltage stepped up by the converter is increased to operate the alternating-current motor in sinusoidal PWM control, a motor loss is reduced. However, due to a switching loss resulting from an increase in the number of switching operations, a loss in the converter and the inverter increases. Thus, a loss of the whole system that includes the alternating-current motor is minimized when the current vector of motor current is on an optimal current advance line at which maximum torque is output or near the optimal current advance line during rectangular wave control.
When the operation of the alternating-current motor is controlled in a rectangular wave control mode in which the current phase of motor current is on the optimal current advance line or near the optimal current advance line in this way (such a current phase is hereinafter referred to as the optimal current phase), a modulation factor in rectangular wave control is constant (for example, 0.78) and therefore, it is not possible to variably control the system voltage while setting a modulation factor as a target as described in JP 2006-311768 A.
In addition, in a motor control system that includes a step-up converter, rectangular wave control with the above-described optimal current phase is executed by directly using a battery voltage as a system voltage without stepping up the battery voltage, the step-up operation of the converter needs to be started at a certain point in time as output torque required for a motor increases. However, because the modulation factor is constant as described above, it is not possible to determine whether to start the step-up operation on the basis of the modulation factor.
In this case, it is conceivable that the step-up operation of the converter is started immediately after shifting into rectangular wave control. In this case, the system voltage after starting step-up operation needs to be increased by at least a minimum step-up voltage having a predetermined value due to restrictions on the configuration of the converter, so an operation point at which rectangular wave control with the optimal current phase is executed by using the battery voltage shifts into an operation point in overmodulation control mode or sinusoidal PWM control mode in which a system loss is relatively large due to the above-described increase in the system voltage by the minimum step-up voltage. As a result, until a torque command to the motor increases and rectangular wave control is executed again after the start of step-up operation of the converter, the system is operated in a state where a system loss is large.